1. Field of the Invention
This invention relates to a method for control of solution concentrations in an absorption chiller so as to prevent crystallization of the absorbent. More particularly, this invention relates to a method for control of solution concentrations in a LiBr/water absorption chiller so as to prevent crystallization of the LiBr/water solution.
Efficiency of the absorption cooling process is a function of solution concentration leaving the generator of the absorption system. Absorbent-refrigerant solution from the absorber enters the generator as a sub-cooled liquid. Energy used to raise the temperature of the absorbent-refrigerant solution to its boiling point is not recoverable. Additional energy added will liberate refrigerant from the solution. This energy is recoverable as machine capacity. By controlling absorbent-refrigerant solution concentration, the energy lost to bring the absorbent-refrigerant solution to its boiling point becomes a smaller fraction of the total energy input. As the amount of refrigerant produced increases, the efficiency of the system increases, as does the concentration of absorbent in the absorbent-refrigerant solution. The limit occurs when the absorbent in the absorbent-enriched solution exiting the heat exchanger in which the absorbent-refrigerant solution to the generator is preheated crystallizes. Controlling the concentration of the absorbent-enriched solution to maintain a fixed temperature margin, defined as the difference between the crystallization temperature of the absorbent-enriched solution and the temperature of the absorbent-refrigerant solution exiting the heat exchanger, maximizes cycle efficiency at all operating conditions.
2. Description of Prior Art
Absorption cooling systems are well established in the prior art. In such systems, an absorbent-refrigerant solution from an absorber is heated in a generator to produce an absorbent-refrigerant vapor. The absorbent-refrigerant vapor is separated and condensed, producing a substantially pure liquid refrigerant and an absorbent-enriched solution, that is a regenerated absorbent-refrigerant solution. The absorbent-enriched solution is circulated through a heat exchanger in heat exchange relationship with the absorbent-refrigerant solution from the absorber, preheating the absorbent-refrigerant solution before its introduction into the generator and producing a reduced temperature absorbent-enriched solution, which reduced temperature absorbent-enriched solution is returned to the absorber. The substantially pure liquid refrigerant is evaporated, producing a substantially pure refrigerant vapor, and the substantially pure refrigerant vapor is absorbed into the reduced temperature absorbent-enriched solution in the absorber, producing the absorbent-refrigerant solution.
As previously stated, efficiency of the absorption cooling process is a function of solution concentration leaving the generator. As the amount of refrigerant produced increases, thereby increasing the efficiency of the process, the concentration of absorbent in the absorbent-enriched solution increases, the limit occurring when the solution crystallizes in the heat exchanger. Several methods and apparatuses for controlling and/or preventing crystallization in an absorption cooling system are taught by the prior art. U.S. Pat. No. 3,604,216 teaches a control switch for preventing crystallization in an absorption refrigeration system, which control switch senses a rise in the temperature of the absorbent-enriched solution leaving the solution heat exchanger, which temperature rise is indicative of impending crystallization. A sensor of the control switch actuates controls to cause dilution of the concentrated solution in the heat exchanger in response to the temperature rise, thereby avoiding crystallization.
U.S. Pat. No. 5,255,534 teaches an absorption refrigeration system that operates around a crystallization curve of a solution with a crystallization limitation in which a portion of the absorbent-refrigerant solution from the absorber is mixed with the absorbent-enriched solution from the generator, the mixture being controlled so that the cycle steps around the crystallization curve of the solution.
Crystallization being a function of solution concentration, solution concentration is calculated using the relationship between solution temperature, refrigerant temperature, and concentration. U.S. Pat. No. 4,572,830 teaches a process for determining and controlling the composition of aqueous solutions of ammonia and carbon dioxide in which the density and saturation temperature of the solution is measured and the ammonia, carbon dioxide, and water composition is determined by comparison to densities and saturation temperatures of solutions of known composition, thereby enabling determination of the crystallization concentration at the crystallization temperature. U.S. Pat. No. 5,246,593 teaches a method for controlling deposition of silica scales from aqueous solutions containing large concentrations of dissolved amorphous silica by measuring concentrations of all dissolved solutes to determine the concentration above which silica will precipitate and processing the data, using the Setchnow equation, to generate a signal which can be used for automatically adjusting system parameters to prevent precipitation.
U.S. Pat. No. 4,676,870 teaches a method and apparatus for maintaining a desired concentration of a dissolved solid in the liquid product stream withdrawn from a multi-effect evaporator using temperature and boiling point rise of the solution to determine concentration of the dissolved solid in the solution. A feedforward control signal manipulates the feed inlet in response to steam flow to the first effect evaporator and concentration of the solution in the first and second effect evaporators.
U.S. Pat. No. 4,269,034 teaches an absorption-refrigeration system having a control system which adjusts operation of the absorption unit thereof on the basis of load demand. The control system monitors the temperature of the chilled fluid entering and leaving the evaporator and provides signals accordingly. A differential temperature controller, which receives the probe signals, compares the signals against each other and against predetermined design specifications for the absorption unit, and provides a resultant signal dependent upon the comparisons. Flow control valves, in turn, receive the resultant signal and regulate the flow of the condenser fluid, the chilled fluid, and the heating medium.
U.S. Pat. No. 5,294,357 teaches a method for preparing salt solutions having desired thermodynamic properties employing a mathematical formulation for determining the water activity in the solution at two different high salt concentrations at a reference temperature, wherein the water activity is related to the mole fraction of the water in the solution.